1. Field of the Invention
This invention relates to voltage reference circuits for providing a constant reference voltage as an output from the circuit, and more particularly, to a voltage reference circuit utilizing the differences in threshold voltage changes with temperature based on a depletion mode MOSFET transistor and an enhancement mode MOSFET transistor.
2. Description of the Prior Art
In the prior art several techniques have been utilized to produce a voltage reference from a power supply subject to variations in voltage and temperature, however none of the techniques have been totally successful. One technique which has been utilized is a voltage divider which is connected across the power supply and the reference voltage is taken from a tap on the voltage divider. This arrangement is not satisfactory for variations in the supply voltage.
A second technique known in the prior art is the utilization of bandgap techniques utilizing bipolar transistors. This type of voltage reference is described in an article entitled "A Floating CMOS Bandgap Reference Voltage for Differential Applications" by M. Ferro et al. in the IEEE Journal of Solid State Circuits, Vol. SC-24, pp. 690-697, June, 1989. Although this type of circuit provides an accurate voltage reference which can be built on a CMOS chip, it is often not used on a CMOS chip because the substrate currents caused by the bipolar transistors may be unacceptable.
A third voltage reference circuit known in the prior art is based on the use of the difference between the threshold voltages of depletion mode and enhancement mode field effect transistors. However, this technique utilizes a circuit which does not permit fine adjustment of the voltage reference value and complete temperature compensation is not possible. A circuit of this type is described in an article entitled "A New NMOS Temperature-stable Voltage Reference" by R. Blauschild, in the IEEE Journal of Solid-State Circuits, Vol. SC-13, pp. 767-773, December 1978. The circuit described in Blauschild et al. is not economically feasible for 5 volt power supplies because special processing of the devices used in the voltage reference circuit is required. The threshold voltage difference in the Blauschild et al. circuitry may be larger than the power supply voltage in some applications, rendering the circuit useless for the application.